I know that fertility drops a lot after 26 wks of age. But does this apply to both male and female quail? Or could you perhaps increase fertility by introducing younger hens to the established males or vice versa?

Yes, your right about male fertility, it drops after 26 weeks old, female fertility also drops at about 30 weeks.

I tried to "spike" some 32 week old females last year (Spike means, remove old males and replace with younger males), and the older hens nearly killed the younger males, I tried this a few time, but the same thing happened.

I now cull all my birds at 26 weeks old, and always plan for other birds to replace them some months before.

One experiment this year I'm doing is:-Trying to determain how many eggs a quail lays in one full year (52 weeks) You read so many different stories, I'd like to do this one myself. ( can a quail really lay 300 eggs in one year?) Watch this space.lol.